Automatic dishwashing (hereinafter ADW) machines employ a variety of wash cycles, or in the case of commercial practice, a variety of machine stages, which usually include a pre-rinse, one or more spray washings using an aqueous detergent solution, and one or more rinses to remove residual detergent and loosened soil. In the majority of modern machines, a rinse aid composition is added, via a separate dispenser, to the final rinse cycle or stage, which composition serves to promote wetting, enhance sheet flow production and increase the rate of water drainage, thereby reducing water spotting on the washed and dried tableware. The rinse aid, which is liquid, contains a low foaming nonionic surfactant and a chelating agent in a hydrotrope-water solubilizing system.
In areas where the water supply has a low level of mineral hardness, i.e., .ltoreq.50 ppm expressed as CaCO.sub.3, or in ADW machines whose water supply is presoftened, it has been noticed that glassware subjected to repetitive washing in an ADW machine develops a surface cloudiness which is irreversible. Under similar treatment conditions, decorated china articles such as plates and dishes also show surface deterioration. These effects often manifest themselves as an iridescent film that displays rainbow hues in light reflected from the surface of the article and the effects become progressively more pronounced with repeated treatment. Whilst the origin of this surface damage has not been definitely established, it is believed that the problem arises from chelating agent carried over from the wash or contained in the rinse aid, attacking the surface during the final rinse or the subsequent drying step.
The effect of detergents on glassware in domestic dishwashers is discussed in a paper entitled "The present position of investigations into the behaviour of glass during mechanical dishwashing" presented by Th. Altenschoepfer in April 1971 at a symposium in Charleroi, Belgium. It had been recognized that the use of metal ions such as zinc in mechanical dishwashing detergent compositions contributes towards the inhibition of corrosion.
Silanes and amino-silanes are widely used in the chemical industry, mostly as coupling agents between inorganic and organic surfaces. These compounds have also found application for metal-surface protection. The protective treatment is applied from an aqueous medium, possibly from solvent systems containing lower alcohols and water, depending upon the characteristics of the silanes. Representative of this state of the art are: U.S. Pat. No. 3,085,908, Morehouse et al., U.S. Pat. No. 3,175,921, Hedlund, and French Pat. No. 1,207,724, Morehouse et al.
The modification of siliceous surfaces for the purpose of conferring various properties is known in the art. Examples include U.S. Pat. Nos. 4,005,118 and 4,005,025 which utilize quaternized amino-silanes to provide soil release properties to vitreous enamel and glass articles when applied from a wash or rinse solution, and U.S. Pat. No. 2,971,865 which employs unquaternized amino-silanes as coupling compounds to attach certain type of dyestuff to glassware.
The preparation of a broad class of gamma-amino-propylalkoxysilanes is known from German Application DOS No. 17 93 280.
None of the above references discuss the corrosion of glass or decorated vitreous enamel ware arising from treatment with a solution of a chelating agent in water of low mineral hardness and close to neutral pH, such as takes place when a conventionally formulated rinse aid is added to the final rinse stage of an ADW machine cycle. It has now surprisingly been found that the addition of certain aminosilanes to the final rinse substantially eliminates this soft water corrosion.